Gas turbine power plants, especially small sized ones, where it is difficult to provide cooling of the turbine, suffer from certain disadvantages, mainly high specific fuel consumption, high costs and specific space requirements. One of the most efficient remedies is to raise the gas temperature, but the strength of conventional, uncooled metallic, heat resistant materials will set a limit to that. Ceramic materials, on the other hand, have a far better capacity to withstand high temperatures, but their strength is generally much less than that of metallic materials. For commercially available ceramic materials, suitable for mass production, the practically useful strength is roughly about one half only of the finest heat resistant metallic materials, taking brittleness and production statistics into account.
Many proposals for making turbine rotors wholly or partly of ceramic materials have been put forward, but have, so far, been no practical succes due to these limitations and to lack of full understanding of the behaviour of ceramic material and statistics from production (Weilbull's number etcetera).
In order to secure simplicity of design the turbine driving the compressor is mounted upon the same shaft as the latter. It is here presupposed that the gas turbine plant is of sufficient advanced design to have at least one further turbine delivering external power and that the components are of conventional turbo type, i.e. centrifugal or axial compressor and axial or radial turbine.
The specific power obtainable from, or consumable by a rotor is roughly proportional to the square of the peripheral speed. If a single, turbine rotor shall produce both compressor and output power the compressor and turbine diameters are, generally speaking, of about the same size.
If the turbine power is split upon two turbine rotors, of which one drives the compressor, both rotors can be designed with smaller diameters, especially the compressor driving turbine, which usually is supplied with the hot gas. This reduces the centrifugal stresses in both turbine rotors, but far from enough to make possible a practical use of present day ceramic materials.
The strength of a reproducible ceramic turbine rotor is so markedly less than that of a rotor of conventional, heat resistant metallic material, that every effect to obtain a single piece ceramic rotor, operating in conventional manner, i.e. at the same speed as the compressor, producing the required power is bound to fail. To adapt the strength of the material to the speed of the rotor it is necessary to substantially reduce the diameter and thus the peripheral speed. This will reduce the stresses imposed upon the material, but also the power obtainable from the rotor. In order to secure the power necessary for driving the compressor means must be provided for supporting the turbine.
The reduced power-generating capacity caused by the "undersized" compressor turbine will largely be compensated by the possibility to drastically increase the temperature. The increased temperature will, however, result in a bigger volume of gas. This can be compensated for by designing the turbine rotor with enlarged vane openings permitting the passage of this bigger gas volume. This will, generally speaking, means that the smaller rotor should have proportionally fewer blades with a higher "aspect ratio" than in the case with a conventional rotor. This is aerodynamically and stress-wise favourable, and can be accepted as there is no necessity to extract a maximum of power in this stage. The possibility to arrange the rotor with an increased spacing between the vanes, and also to design the same so the gas stream will pass through the rotor with small change in its angular direction, will result in a reduced resistance to flow. One way of reducing the necessary change of direction also in a stator preceeding the rotor is to locate the combustion chamber excentrically with respect to a longitudinal plane through the rotor axis, and to arrange the turbine stator in a volute chamber, whereby the gases, already when entering the stator will have a certain amount of co-rotation. This will mean a drastic reduction in size and number of blades, both in the rotor and in the stator, which for some applications can now be made without vane overlap, facilitating manufacture and reducing costs and weight.